The present invention relates to a method of fastening a tool in a tool holder, the tool comprises a front side shaped for use in cold forming of metal wire into nails, screws, etc. The tool holder comprises a recess shaped for receiving the tool. The invention is directed to a tool system comprising a tool and a tool holder, and further to the use of a tool and a tool holder.
Other methods of fastening a tool in a tool holder and devices with such purpose have formerly been employed. In EP 406,202 A2 a clamping device for making nails is disclosed, where a die (tool) is mounted in a die holder. The die holder is provided with an opening for receiving the die. The back side of the opening is provided with an extension to obtain a slight elasticity, and across said extension a bolt is mounted to clamp the die in the die holder. However, the force from the bolt is relatively small compared to the forces exerted on the die during the nail making process. The tensile stresses in the die are relatively large, and over the life span of the die numerous stresses are applied and relieved such that the well known phenomena of metal fatigue appears. This often to cracked dies which have to be replaced, leading to extra costs and lost production output due to lost time.
The problem is addressed in EP 870,558 A2 and the corresponding U.S. Pat. No. 5,979,216, where the tool insert (die/tool) has been divided in two parts in order to remove harming effects of tensile stresses, i.e., providing a crack in a determined position. The solution is costly, since more parts, which must fit accurately together, have to be made.
Also another phenomena known as fretting may appear, which leads to deterioration of the tool inserts. The presence of fretting is caused due to the bolt, which is also employed in this technique, being inadequate to provide sufficient clamping force. With inadequate clamping force, the two parts on each side of the artificial crack are moving slightly away from each other during each loading cycle, and back when the load is removed. The slight movement causes wear and consequently fretting.
The basic problem has not been solved, namely that the tool insert (die/tool) is subject to tensile stresses that are too high. When cold forming a wire, the shaping portion of the tool is subject to a high pressure which is substantially radially directed on the concave shaping portion of the tool. The pressure results in compressive stresses near the surface, where the contact is between the tool and the wire. However, slightly further away from the surface, tangentially oriented to the concave shaping portion, tensile stresses prevail.
In case of a flat shaping portion, the pressure also results in compressive stresses near the surface, where the contact is between the tool and the wire. Slightly further away from the surface, tangentially oriented to the origin of contact, tensile stresses also prevail.
One object of the present invention is to provide a method of fastening a tool in a tool holder, such that compressive stresses are initially present in the tool (built-in) when the tool is subjected to the tensile stresses caused by the nail/screw making process. Such method will reduce the resulting tensile stresses.
Another object is to improve the life span of the tool and further to reduce down time in the production.
The new and inventive aspects of the method according to the invention comprise a tool with two opposite sides forming a wedge-shape which are narrowing towards the front side of the tool. The recess in the tool holder comprises two corresponding sides which are placed substantially in the same wedge-shape. The tool has a back side and is fastened by placing the tool in the recess and displacing the tool substantially in the direction of the front side. Then the two opposite sides of the tool are compressed against the corresponding sides of the recess by applying and maintaining pressure on the back side of the tool with fastening means.
By the new and inventive aspects it is obtained, that the tool is compressed between the two opposite sides of the recess, such that compressive stresses are introduced in the tool. Due to the wedge-shape of the tool and the recess, the amount of pressure applied to the back side is amplified several times on the sides of the tool, whereby the compressive stresses reach a high level introduced and maintained by the fastening means. When the tool is afterwards used in the nail/screw making process, the tensile stresses caused thereby, must relieve or overcome the compressive stresses before a state of tensile stress in the tool can be present. Hence, the resulting tensile stress level is at least partly reduced, and may be avoided.
The two opposite sides may further be formed with a wedge-shape relative to a bottom side of the recess. The wedge-shape is narrowing away from the bottom side. The two corresponding sides in the recess in the tool holder are placed in substantially the same wedge-shape, and the tool is fastened against the bottom side of the recess by applying and maintaining pressure on the back side with fastening means. A considerable compression may be obtained similarly to the effect obtained as mentioned and explained above, however in a substantially perpendicular direction. The perpendicular direction is parallel to the direction of another force present in the nail/screw making process. This other force is caused when forming the flat head on the nail or screw. The force introduces shear as well as tensile stresses near and on the surface portion of the tool in contact with the metal wire. A prestressed compression will also reduce the maximum tensile stresses and improve the life span of the tool.
The fastening means may have a back side that is sloped relative to the bottom side of the recess. A wedge with a hole can be placed with one side against the back side. A bolt is connected to the tool holder through the hole pressure may be applied and maintained on the back side by tightening the bolt against the wedge. Hereby a way of amplifying the force from the bolt is obtained, which also further amplifies the compression in the tool.
The new and inventive aspects of the tool system, according to the invention, include the tool having two opposite sides forming a wedge-shape, which narrows towards the front side of the tool. The recess in the tool holder comprises two corresponding sides which are placed substantially in the same wedge-shape. The tool also has a back side that is sloped relative to the bottom side of the recess. A wedge is placed with one side against the back side and the opposite side against an end side of the recess. The wedge comprises a hole through which a bolt is connected to the tool holder. The tool is fastened by tightening the bolt against the wedge.
The tool is compressed between the two opposite sides of the recess, such that compressive stresses are introduced in the tool. Due to the wedge-shape of the two sides, the pressure applied to the back side is amplified several times, whereby the compressive stresses reach a high level. Also the force from the bolt is amplified thereby allowing the compressive stresses to be further increased. An advantage of this system is that the tool may be replaced without having to disconnect the tool holder from the machine since the tool and the fastening means may all be reached from the same accessible side.
The two opposite sides may further be formed with a wedge-shape relative to the bottom side of the recess. The wedge-shape is narrowing away from the bottom side. The two corresponding sides in the recess in the tool holder are placed in substantially the same wedge-shape. As such, the harmful effects from the shear as well as tensile stresses near the contact portion of the tool from the making of the nail/screw head may be reduced. A prestressed state of compression will also reduce the maximum tensile stresses and improve the life span of the tool.
The angle of the wedge-shape narrowing towards the front side of the tool may be between 0.5 and 45 degrees in an alternative embodiment of the present invention.
In yet another embodiment, the angle may be between 1 and 15 degrees
In a further embodiment, the angle of the wedge-shape narrowing away from the bottom side of the recess may be between 1 and 30 degrees.
The new and inventive aspects of the tool according to the invention are the two opposite sides forming a wedge-shape which narrows towards the front side of the tool. The tool is configured for fastening in a tool holder having a recess adapted for receiving the tool. The tool also has fastening means for fastening the tool by application and maintenance of pressure on the tool in a way such that the two opposite sides may be pre-stressed in the recess. The tool may be fastened in a tool holder so that tensile stresses in the tool are reduced and the life span of the tool is increased.
The tool holder is adapted for receiving and fastening a wedge-shaped tool for use in cold forming of metal wire into nails, screws, etc. The tool holder comprises a recess shaped for receiving the tool. The recess in the tool holder has two sides forming a wedge-shape which narrows towards an open end of the recess. The tool holder has fastening means configured for fastening the tool by application and maintenance of pressure on the tool, in a way such that the tool may be pre-stressed in the recess against the sides. As such, the tool holder may receive and fasten a tool, in a way such that the life span of the tool is increased.
The tool and the tool holder are used in a machine or plant in a process for the manufacture of nails, screws and similar items so as to decrease production time.